1. Field of the Invention
The present invention relates to sheet metal forming processes and tooling for reducing the effect of spring-back on formed panels.
2. Background Art
Sheet metal is generally formed in a sheet metal forming process in which a sheet metal blank is drawn to an initial shape, stamped, flanged, formed and pierced in a series of steps. Spring-back occurs as a result of bending moments that develop in the blank as the sheet metal is formed to the desired shape. Spring-back causes the panels to partially return to a prior shape after a panel is formed in a sheet metal die or other sheet metal forming process.
New types of materials have been proposed for making sheet metal parts with higher strength and lower weight. Specialized steels and aluminum sheets are available that offer high strength and low weight which is desirable in many applications. Many high strength and low weight metals are subject to increased spring-back after forming.
One approach to compensating for spring-back is to predict spring-back in the die design process. The shape of the die may be modified to compensate for spring-back.
Another approach to compensating for spring-back is to stretch the formed blank to eliminate bending moments in the blank. If a part is to be stretched to reduce spring-back, the depth of draw must be limited to permit the stretching operations to adequately compensate for spring-back.
The degree of spring-back may vary from coil to coil. Some coils have only limited spring-back, while other coils even of the same grade or alloy may have greater spring-back. Spring-back is also affected by the extent of wear of the sheet metal forming dies. Increased spring-back may occur when the dies become worn.
The above problems are addressed by Applicant's invention as summarized below.